Heterotrimeric G proteins function to relay information from cell surface receptors to intracellular effectors. Each of a very broad range of receptors specifically detects an extracellular stimulus (a photon, pheromone, odorant, hormone or neurotransmitter) while the effectors (i.e., adenyl cyclase), which act to generate one or more intracellular messengers, are less numerous. In mammals, G protein Alpha, Beta and Gamma polypeptides are encoded by at least 16, 4 and 7 genes, respectively. Most interest in G proteins has been focused on their Alpha subunits, since these proteins bind and hydrolyze GTP and most obviously regulate the activity of the best studied effectors. Four distinct classes of G Alpha subunits have been identified; these include G Alpha s, G Alpha i, G Alpha q and G Alpha 12/13. The two members of the fourth class of G Alpha subunit proteins, G Alpha 12 and G Alpha 13, are insensitive to ADP-ribosylation by pertussis toxin, share 67% identity with each other and less than 45% identity with other G Alpha subunits and are widely expressed in a broad range of tissues.